The use of aromatic diamines as chain extenders in the production of polyurethanes is known. In order to ensure reasonable processing times, reactive aromatic isocyanates, which are the most widely used in practice, are preferably reacted with sluggishly reacting diamines. The sluggishly reacting diamines used include aromatic diamines wherein the basicity, and therefore, the reactivity towards isocyanates, has been reduced by the introduction of halogen or carboxy substituents. 3,3'-Dichloro-4,4'-diaminodiphenyl methane (MOCA), which hitherto has been the most widely used, is an example of one such aromatic diamine. However, the main disadvantage of this compound is its toxicity. U.S. Pat. No. 3,823,833 discloses using 2,2'- or 4,4'- diaminodiphenyl sulphide as chain extender in the production of polyurethane systems. However, the disadvantages of these compounds are the susceptibility of the S-S-group in the polyurethane segment to cleavage by reduction or oxidation, the short pot lives, and the long in-mold times during the reaction with polyisocyanates, which preclude economic processing.
Published Japanese Specification No. 9195/70 describes a concentrated polyurethane solution which may be used for the production of fibers, films or artificial leather. The chain extender used is a diamine of the formula NH.sub.2 -R-(SR).sub.n -NH.sub.2 where R is an aliphatic, aromatic, alicyclic or heterocyclic group and n=1 or 2. The main disadvantage of a system extended with diamines such as these is the fact that the excessive reactivity of the amino groups to isocyanates necessitates the use of an organic solvent which considerably increases the costs, necessitates additional extraction equipment, and involves the danger of fire and explosions.
In U.S. Pat. No. 3,920,617, it is proposed to use sulfur-containing polyamines corresponding to the general formula ##STR3## as chain extenders in the production of polyurethane elastomers. This type of compound provides for a sufficiently long processing time at temperatures in the range from 25.degree. to 60.degree. C. However, the elastomers obtained are inadequately crosslinked and inhomogeneous because of the excessive viscosity of NCO-prepolymers at processing temperatures used. Although homogeneous elastomers are obtained at processing temperatures in the range of 80.degree. to 110.degree. C., the pot life is only slightly increased in comparison with MOCA, with the result that the processing conditions are not significantly improved when compared to the aromatic diamines of the prior art.
An object of the present invention is to obviate the above-described disadvantages attending conventional aromatic diamine chain extenders, such as toxicity and ready cleavage of the S-S-group by reduction or oxidation, and the polyurethane elastomers obtainable therefrom and, additionally, to provide reactive polyurethane systems which have the advantage of solvent-free processing, longer pot lives and shorter in-mold times.